Functional soft materials and methods of making and using thereof

ABSTRACT

Disclosed are functional materials for use in additive manufacturing (AM). The functional material can comprise an elastomeric composition (e.g., a silicone composite) for use in, for example, direct ink writing. The elastomeric composition can include and elastomeric resin, and a magnetic nanorod filler dispersed within the elastomeric resin. Nanorod characteristics (e.g., length, diameter, aspect ratio) can be selected to create 3D-printed constructs with desired mechanical properties along different axes. Furthermore, since nickel nanorods are ferromagnetic, the spatial distribution and orientation of nanorods within the continuous phase can be controlled with an external magnetic field. This level of control over the nanostructure of the material system offers another degree of freedom in the design of functional parts and components with anisotropic properties. Magnetic fields can be used to remotely sense compression of the constructs, or alternatively, control the stiffness of these materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to 62/858,946 filed Jun.7, 2019, the disclosure of which is incorporated herein by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-NA0002839awarded by the Department of Energy. The government has certain rightsin the invention.

BACKGROUND

Soft materials are often composed of polymers or other flexiblestructural components. Fillers or other materials are commonly added tothe soft material to facilitate processing (e.g., additivemanufacturing) and/or to achieve desirable properties of the finalproduct. For example, silicone polymers can be printed in 3D when silicafillers are included at sufficient concentration to provide structuralfidelity to the printed material. This step is often followed by curingto yield a robust product.

Soft materials have a wide range of commercial uses, includingapplications in consumer products and as devices in the healthcareindustry. However, next generation materials would benefit from theability to remotely sense changes in soft materials. Remote or‘wireless’ sensing would allow monitoring of the device performance andfidelity, for example. In addition, a means to remotely control themechanical properties of soft materials would also prove beneficial.Remotely controlling to stiffen or soften as needed for a particularsituation would enable new applications.

SUMMARY

Remote sensing and/or remote control of soft materials can be achievedby incorporating an additive that can be detected and/or influenced byan external force. A variety of detectable/controllable phenomena (e.g.,thermal, optical, electric, acoustic, and magnetic fields) can beutilized. For example, the inclusion of graphene can improve theconductivity of soft materials and changes in the material structure canbe detected as a change in the resistance of the material.

While many of these approaches require wired connections, magneticfields offer a unique opportunity for wireless detection and/or controlof soft materials. For example, ferromagnetic nanoparticles can besuspended in a precursor polymer and 3D printed to develop high fidelitystructures. Change in the magnetic field of the nanoparticles caused bydeformation of the soft material can be detected by sensors such as aHall Effect sensor (e.g., a magnetometer). In addition, an externalmagnetic field can be applied to stiffen or soften the material, forexample. If desired, a magnetometer and electromagnet can both beinterfaced with the soft material. Such a system can be operated in aclosed-loop fashion, such that forces acting on the soft material can bemeasured in real time, and an external magnetic field can be applied inresponse to alter the mechanical properties of the soft material inresponse. The compositions described herein can be used to help predictpossible injuries from repeated stress and strain within people andpotentially professional athletes. In some embodiments, the compositioncan be integrated within a shoe sole/insole for diagnosing andmonitoring in patients with an increased risk or undergoing treatmentfor neuropathy.

DESCRIPTION OF DRAWINGS

FIG. 1 is a plot showing the average nickel nanorod length (n=50) as afunction of electrochemical deposition time.

FIGS. 2A-2H so the characterization of electrodeposited nanorods. FIGS.2A-2B show TEM images of nanorods. FIG. 2C shows the elementalcomposition of nanorods ascertained from energy dispersive x-rayspectroscopy (EDX). FIGS. 2D-2H show dark-field STEM image withcorresponding element maps of nanorods.

FIGS. 3A-3C illustrate the viscoelastic properties of NuSil R40-2181containing nickel nanorods (1% by weight) with medium and large lengths(medium=4-8 μm; large=8-12 μm). Representative continuous flow curves(FIG. 3A), oscillatory stress sweep curves (FIG. 3B), and stressrecovery curves (FIG. 3C) of NuSil R40-2181 formulations are shown.

FIG. 4 includes optical macro- and microscopic images of 3D-printedNuSil R40-2181 containing various concentrations of nickel nanorods (0,0.1, 1 and 10% by weight). Scale bars=2 mm (top), 200 μm (middle), and50 μm (bottom).

FIGS. 5A-5B show compressive moduli of 3D-printed NuSil R40-2181structures containing 1% by weight vertically aligned medium nickelnanorods (FIG. 5A) and 1% by weight horizontally aligned nickel nanorodsof various lengths (small=1-4 μm; medium=4-8 μm; large=8-12 μm) (FIG.5B).

FIGS. 6A-6B show bright field microscopy images (20× objective) ofcryo-sectioned SYLGARD 184 PDMS-nanorod composites with vertically (FIG.6A) and horizontally (FIG. 6B) oriented nanorods. Scale bar=5 μm.

FIGS. 7A-7B show the compressive moduli of SYLGARD 184 PDMS structurescontaining 1% by weight vertically aligned nickel nanorods of variouslengths (small=1-4 μm; large=8-12 μm) (FIG. 7A) and 1% by weightvertically and horizontally aligned large nickel nanorods (FIG. 7B)exposed to an electromagnetic field in increasing intervals up to 272Gauss.

FIGS. 8A-8B illustrate the remote sensing of compressive forces bymeasurement of magnetic field strength of PDMS-nanorod composites (1 wt% nanorods) (FIG. 8A) and NuSil R40-2181-nanorod composites (1 wt %nanorods) (FIG. 8B) during sample compression with DMA. The samples werecompressed atop a triple-axis magnetometer, which was used with amicrocontroller to record the magnetic field strength value while thesamples were subjected to compressive forces in 100 gram intervals up to600 grams.

FIG. 9 illustrates the 3-dimensional shape of various classes ofparticles: (a) spheres, (b) rectangular disks, (c) high aspect ratiorectangular disks, (d) rods, (e) high aspect ratio rods, (f) worms, (g)oblate ellipses (h) prolate ellipses, (i) elliptical disks, (j) UFOs,(k) circular disks, (l) barrels, (m) bullets, (n) pills, (o) pulleys,(p) bi-convex lenses, (q) ribbons, (r) ravioli, (s) flat pills, (t)bicones, (u) diamond disks, (v) emarginate disks, (w) elongatedhexagonal disks, (x) tacos, (y) wrinkled prolate ellipsoids, (z)wrinkled oblate ellipsoids and (aa) porous elliptical disks.

FIG. 10 show changes in magnetic field strength in 1 wt % horizontal andvertically aligned PDMS-nickel nanorods constructs at variouscompression intervals.

FIG. 11 show the magnetic field response from the x, y, and z axis of a1 wt % PDMS-nickel nanorod construct during different magnitudes ofdisplacement.

FIG. 12 show extrusion tests of nickel nanorods in NuSil R40 at 0.1, 1,and 10 wt %. The scale bars are equal to 20 μm. 13

FIG. 13 show microscope imaging of 1 wt % (small=1-4 μm [left],medium=4-8 μm [middle], and large=8-12 μm [right]) nickel nanorods inNuSil R40.

FIG. 14A-14C illustrates the experimental setup of compression testing.A) Cylindrical sample is placed on top of magnetometer and top geometryof ElectroForce 5500 is lowered to sample height until contact is made.B) Theoretical graph of force (g) versus time (s) showing theincremental steps of 200 g of force applied at 20 g/s for steps i, ii,and iii, and the removal of 200 g of force applied during steps iv, v,and vi. C) Theoretical figure of a cylinder being compressed orrecovered during each incremental step of applied force over the courseof 140 s.

FIG. 15A-15B illustrates A) Custom built stepper motor to performpreliminary studies B) Compression geometry of stepper motor i) NEMA-232 phase stepper motor ii) Motor controller with A4988 stepper motordriver and iii) LSM303 triple-axis accelerometer and magnetometer iv)Sensor controller v) Cylindrical sample to be compressed.

FIG. 16A-16F show images of A) Nickel nanorods, B) carbonyl ironmicrospheres, C) cobalt nanowires D) iron(II,III) oxide (FeO and Fe₂O₃)nanopowder, E) magnetite (Fe₃O₄) powder F) neodymium iron boron powder.The scale bars are equal to 10 mm.

FIG. 17A-17F show all six ferromagnetic materials at 0.01 wt % (Left),0.1 wt % (Middle), and 1 wt % (Right) within PDMS. A) Nickel, B)carbonyl iron C) cobalt D) iron oxide, E) magnetite, and F) neodymium.

FIG. 18A-18B show images of A) Blank samples of 2-part polyurethane(Left), DMS-V21 (Middle-Left), DMS-V33 (Middle-Right), and PDMS (Right).B) Samples each containing 1 wt % iron oxide.

FIG. 19 show the magnetic field strength (displayed from the z-axis) andthe force versus time of a 1 wt % sample of nickel.

FIG. 20 show the magnetic field strength from all six magnetic fillersat 1 wt % during 0, 200, 400, and 600 g of force.

FIG. 21 show the magnetic field strength (displayed from the z axis) andthe displacement versus time of the same 1 wt % sample of nickeldepicted in FIG. 19.

FIG. 22 is a bar graph showing the displacement of all six magneticfillers at 1 wt % during 0, 200, 400, and 600 g of force.

FIG. 23 show displacement as a function of force of the same 1 wt %nickel sample seen in FIG. 19.

FIG. 24 show displacement as a function of force for one sample (n=1) ofeach magnetic material at 1 wt % within PDMS. Only the compressioncycles are displayed here.

FIG. 25 show magnetic field strength as a function of displacement forone sample (n=1) of each magnetic material at 1 wt % within PDMS. Onlythe compression cycles are displayed here.

FIG. 26 show preliminary results of field strength (displayed from the zaxis) and the displacement versus time of 1 wt % nickel.

FIG. 27 is a bar graph showing magnetic field strength from materialsstudied during preliminary trials at 1 wt % during 0, 0.75, 1.50, and2.25 mm of displacement.

FIG. 28 is a bar graph showing magnetic field strength of all sixmagnetic fillers at 0.01, 0.1, and 1 wt % during 600 g of force.

FIG. 29 is a bar graph showing displacement of all six magnetic fillersat 0.01, 0.1, and 1 wt % during 600 g of force.

FIG. 30 is a bar graph showing magnetic field strength of blank softmaterials and soft materials with 1 wt % iron oxide during 0, 200, 400,and 600 g of force.

FIG. 31 is a bar graph showing displacement of blank soft materials andsoft materials with 1 wt % iron oxide during 0, 200, 400, and 600 g offorce.

FIG. 32 show displacement as a function of force for one sample (n=1) ofeach soft material with and without 1 wt % iron oxide. Only thecompression cycles are displayed here.

FIG. 33 show the estimated settling velocities of magnetic particleswithin solutions of varying viscosities.

FIG. 34 show the estimated settling velocities of differing particlesizes (1, 25, 50, 75, and 100 mm) of neodymium within solutions ofvarying viscosities.

FIG. 35 is a bar graph showing magnetic field strength from all sixmagnetic fillers at 0.1 wt % during 0, 200, 400, and 600 g of force.

FIG. 36 is a bar graph showing displacement of all six magnetic fillersat 0.1 wt % during 0, 200, 400, and 600 g of force.

FIG. 37 is a bar graph showing magnetic field strength from all sixmagnetic fillers at 0.01 wt % during 0, 200, 400, and 600 g of force.

FIG. 38 is a bar graph showing displacement of all six magnetic fillersat 0.01 wt % during 0, 200, 400, and 600 g of force.

FIG. 39 show the magnetic field strength as a function of force for onesample (n=1) of each magnetic material at 1 wt % within PDMS. Only thecompression cycles are displayed here.

DETAILED DESCRIPTION General Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this disclosure belongs.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. By “about” is meant within10% of the value, e.g., within 9, 8, 8, 7, 6, 5, 4, 3, 2, or 1% of thevalue. When such a range is expressed, another aspect includes from theone particular value and/or to the other particular value. Similarly,when values are expressed as approximations, by use of the antecedent“about,” it will be understood that the particular value forms anotheraspect. It will be further understood that the endpoints of each of theranges are significant both in relation to the other endpoint, andindependently of the other endpoint. It is also understood that thereare a number of values disclosed herein, and that each value is alsoherein disclosed as “about” that particular value in addition to thevalue itself. For example, if the value “10” is disclosed, then “about10” is also disclosed.

The term “comprising” and variations thereof as used herein is usedsynonymously with the term “including” and variations thereof and areopen, non-limiting terms. Although the terms “comprising” and“including” have been used herein to describe various embodiments, theterms “consisting essentially of” and “consisting of” can be used inplace of “comprising” and “including” to provide for more specificembodiments and are also disclosed. Throughout the description andclaims of this specification the word “comprise” and other forms of theword, such as “comprising” and “comprises,” means including but notlimited to, and is not intended to exclude, for example, otheradditives, components, integers, or steps.

As used in the specification and claims, the singular form “a”, “an”,and “the” include plural references unless the context clearly dictatesotherwise. For example, the term “an agent” includes a plurality ofagents, including mixtures thereof.

As used herein, the terms “may,” “optionally,” and “may optionally” areused interchangeably and are meant to include cases in which thecondition occurs as well as cases in which the condition does not occur.Thus, for example, the statement that a formulation “may include anexcipient” is meant to include cases in which the formulation includesan excipient as well as cases in which the formulation does not includean excipient.

A “decrease” can refer to any change that results in a smaller amount ofa symptom, disease, composition, condition, or activity. A substance isalso understood to decrease the genetic output of a gene when thegenetic output of the gene product with the substance is less relativeto the output of the gene product without the substance. Also, forexample, a decrease can be a change in the symptoms of a disorder suchthat the symptoms are less than previously observed. A decrease can beany individual, median, or average decrease in a condition, symptom,activity, composition in a statistically significant amount. Thus, thedecrease can be a 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40,45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100% decrease so long asthe decrease is statistically significant.

“Inhibit,” “inhibiting,” and “inhibition” mean to decrease an activity,response, condition, disease, or other biological parameter. This caninclude but is not limited to the complete ablation of the activity,response, condition, or disease. This may also include, for example, a10% reduction in the activity, response, condition, or disease ascompared to the native or control level. Thus, the reduction can be a10, 20, 30, 40, 50, 60, 70, 80, 90, 100%, or any amount of reduction inbetween as compared to native or control levels.

By “reduce” or other forms of the word, such as “reducing” or“reduction,” is meant lowering of an event or characteristic (e.g.,tumor growth). It is understood that this is typically in relation tosome standard or expected value, in other words it is relative, but thatit is not always necessary for the standard or relative value to bereferred to. For example, “reduces tumor growth” means reducing the rateof growth of a tumor relative to a standard or a control.

As used herein, the terms “treating” or “treatment” of a subjectincludes the administration of a drug to a subject with the purpose ofpreventing, curing, healing, alleviating, relieving, altering,remedying, ameliorating, improving, stabilizing or affecting a diseaseor disorder, or a symptom of a disease or disorder. The terms “treating”and “treatment” can also refer to reduction in severity and/or frequencyof symptoms, elimination of symptoms and/or underlying cause, preventionof the occurrence of symptoms and/or their underlying cause, andimprovement or remediation of damage.

By “prevent” or other forms of the word, such as “preventing” or“prevention,” is meant to stop a particular event or characteristic, tostabilize or delay the development or progression of a particular eventor characteristic, or to minimize the chances that a particular event orcharacteristic will occur. Prevent does not require comparison to acontrol as it is typically more absolute than, for example, reduce. Asused herein, something could be reduced but not prevented, but somethingthat is reduced could also be prevented. Likewise, something could beprevented but not reduced, but something that is prevented could also bereduced. It is understood that where reduce or prevent are used, unlessspecifically indicated otherwise, the use of the other word is alsoexpressly disclosed. For example, the terms “prevent” or “suppress” canrefer to a treatment that forestalls or slows the onset of a disease orcondition or reduced the severity of the disease or condition. Thus, ifa treatment can treat a disease in a subject having symptoms of thedisease, it can also prevent or suppress that disease in a subject whohas yet to suffer some or all of the symptoms. As used herein, the term“preventing” a disorder or unwanted physiological event in a subjectrefers specifically to the prevention of the occurrence of symptomsand/or their underlying cause, wherein the subject may or may notexhibit heightened susceptibility to the disorder or event.

A “control” is an alternative subject or sample used in an experimentfor comparison purposes. A control can be “positive” or “negative.”

As used herein, by a “subject” is meant an individual. Thus, the“subject” can include domesticated animals (e.g., cats, dogs, etc.),livestock (e.g., cattle, horses, pigs, sheep, goats, etc.), laboratoryanimals (e.g., mouse, rabbit, rat, guinea pig, etc.), and birds.“Subject” can also include a mammal, such as a primate or a human. Thus,the subject can be a human or veterinary patient. The term “patient”refers to a subject under the treatment of a clinician, e.g., physician.

Composition

Disclosed herein are compositions that comprise an elastomeric resin;and a population of anisotropic magnetic particles dispersed within theelastomeric resin.

Anisotropic Magnetic Particles

In some embodiments, the anisotropic magnetic particles can comprisenanoparticles. The term “nanoparticle,” as used herein, generally refersto a particle of any shape having one or more dimensions ranging from 1nm up to, but not including, 1 micron. In some embodiments, thenanoparticles can comprise a particle of any shape having one or moredimensions ranging from 1 nm up to, but not including, 1 micron; and oneor more dimensions of 1 micron or more (e.g., from 1 micron to 10microns, from 1 micron to 20 microns, from 1 micron to 25 microns, orfrom 1 micron to 50 microns).

In other embodiments, the anisotropic magnetic particles can comprisemicroparticles. The microparticles can be of any shape, and have one ormore dimensions ranging from 1 micron to 100 microns. In someembodiments, all dimensions can range from 1 micron to 100 microns.

In some embodiments, the population of anisotropic magnetic particles isa monodisperse population of anisotropic magnetic particles. In otherembodiments, the population of anisotropic magnetic particles is apolydisperse population of anisotropic magnetic particles. In someinstances where the population of anisotropic magnetic particles ismonodisperse, greater that 50% of the particle size distribution, morepreferably 60% of the particle size distribution, most preferably 75% ofthe particle size distribution lies within 10% of the median particlesize.

The anisotropic magnetic particles can comprise any suitable magneticmaterial, such as ferromagnetic alloys comprising Fe, Co, Ni, orcombinations thereof. In certain embodiments, the anisotropic magneticparticles can comprise Ni particles. Such particles can be formed usingmethods known in the art, including synthesis driven by appropriateshaping ligands, template-assisted synthesis, template-assistedelectrodeposition, and magnetically directed assembly. Examples of suchmaterials are described, for example, in Lisjak et al. “AnisotropicMagnetic Nanoparticles: A Review of their Properties, Synthesis, andPotential Applications,” Progress in Materials Science, 2018, 95;286-328 (which is hereby incorporated by reference in its entirety forits description of anisotropic magnetic particles, and which is attachedto this filing).

The anisotropic magnetic particles can be essentially homogeneousthroughout, meaning that the composition does not vary throughout theparticle cross-section (from the particle surface to the particlecenter). Alternatively, the anisotropic magnetic particles can possess anon-homogeneous structure. For example, the particles may possess acore-shell structure, or a multilayer structure (e.g., a magnetic corecoated by a non-magnetic shell material).

The anisotropic magnetic particles may have any desired shape. Incertain embodiments, the particles can have a non-spherical shape. Asgenerally used herein, “non-spherical” is used to describe particleshaving at least one dimension differing from another dimension by aratio of at least 1:1.10. In one embodiment, the non-spherical particleshave at least one dimension which differs from another dimension by aratio of at least 1:1.25. A wide variety of shapes are considered“non-spherical” shapes. For example, as shown in FIG. 9, non-sphericalparticles may be in the shape of rectangular disks, high aspect ratiorectangular disks, rods, high aspect ratio rods, worms, oblate ellipses,prolate ellipses, elliptical disks, UFOs, circular disks, barrels,bullets, pills, pulleys, bi-convex lenses, ribbons, ravioli, flat pill,bicones, diamond disks, emarginated disks, elongated hexagonal disks,tacos, wrinkled prolate ellipsoids, wrinkled oblate ellipsoids, orporous elliptical disks. Additional shapes beyond those illustrated inthe figures are also within the scope of the definition for“non-spherical” shapes.

In some embodiments, the anisotropic magnetic particles can compriserod-shaped particles. “Rod-shaped,” as used herein, refers to a particlewhich has an elongated spherical or cylindrical shape (e.g., the shapeof a pill) or a flattened rod-shape, such as the shape of a green bean.Rod-shaped particles have an aspect ratio of at least 1.25 (e.g., atleast 1.5, at least 2, at least 2.5, or at least 5). “Aspect ratio,” asused herein, refers to the length divided by the diameter of a particle.

In certain embodiments, the particles can be rod-shaped. In someembodiments, the rod-shaped particles can have an aspect ratio, definedas the length of the rod-shaped particle divided by the diameter of therod-shaped particle, of at least 1.25 (e.g., at least 2.5, at least 5,at least 10, at least 15, at least 25, at least 50, at least 100, atleast 150, at least 200, at least 250, or more). In some embodiments,the rod-shaped particles can have an aspect ratio, defined as the lengthof the rod-shaped particle divided by the diameter of the rod-shapedparticle, of 500 or less (e.g., 250 or less, 200 or less, 150 or less,100 or less, 50 or less, 25 or less, 15 or less, 10 or less, 5 or less,or 2.5 or less).

The rod-shaped particles can have an aspect ratio ranging from any ofthe minimum values described above to any of the maximum valuesdescribed above. In certain embodiments, the rod-shaped particles canhave an aspect ratio of from 1.25 to 500 (e.g., from 5 to 500, from 5 to250, from 5 to 100, from 5 to 500, from 5 to 250, or from 5 to 100).

In some embodiments, the rod-shaped particles can have an averagediameter of at least 5 nm (e.g., at least 25 nm, at least 50 nm, atleast 100 nm, at least 200 nm, at least 300 nm, at least 400 nm, atleast 500, at least 600 nm, at least 700 nm, at least 800 nm, or atleast 900 nm). In some embodiments, the rod-shaped particles can have anaverage diameter of 950 nm or less (e.g., 900 nm or less, 800 nm orless, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less,300 nm or less, 200 nm or less, 100 nm or less, 50 nm or less, or 25 nmor less.

The rod-shaped particles can have an average diameter ranging from anyof the minimum values described above to any of the maximum valuesdescribed above. In certain embodiments, the rod-shaped particles canhave an average diameter of from 50 nm to 800 nm (e.g., from 50 nm to500 nm, or from 100 nm to 300 nm).

In some embodiments, the rod-shaped particles can have an average lengthof at least 500 nm (e.g., at least 1 micron, at least 5 microns, atleast 10 microns, at least 15 microns, at least 20 microns, at least 25microns, at least 50 microns, at least 75 microns, at least 100 microns,at least 150 microns, or at least 200 microns). In some embodiments, therod-shaped particles can have an average length of 250 microns or less(e.g., 200 microns or less, 150 microns or less, 100 microns or less, 75microns or less, 50 microns or less, 25 microns or less, 20 microns orless, 15 microns or less, 10 microns or less, 5 microns or less, or 1micron or less).

The rod-shaped particles can have an average length ranging from any ofthe minimum values described above to any of the maximum valuesdescribed above. In certain embodiments, the rod-shaped particles canhave an average length of from 500 nm to 100 microns (e.g., from 1micron to 25 microns).

The anisotropic magnetic particles can be present in the composition inan amount of from 0.1% by weight to 10% by weight (e.g., from 0.1% byweight to 5% by weight, from 0.1% by weight to 2.5% by weight, from 0.1%by weight to 2% by weight, from 0.1% by weight to 1.5% by weight, orfrom 0.1% by weight to 1% by weight), based on the total weight of thecomposition.

The anisotropic magnetic particles can be present in the composition inan amount of from 0.01% by volume to 2.0% by volume (e.g., from 0.01% byvolume to 1.5% by volume, from 0.01% by volume to 1.0% by volume, from0.01% by volume to 0.75% by volume, from 0.01% by volume to 0.5% byvolume, from 0.01% by volume to 0.2% by volume, or from 0.01% by volumeto 0.15% by volume), based on the total volume of the composition.

In some embodiments, the magnetic particles can be uniformly dispersedthroughout the elastomeric resin. In other embodiments, the magneticparticles can by non-homogenously dispersed throughout the elastomericresin. For example, the magnetic particles can be at varyingconcentrations throughout the elastomeric resin (e.g., at a higherconcentration at a region in proximity to a magnetometer and at a lowerconcentration at a region further away from a magnetometer). In someembodiments, a gradient of magnetic particles can be dispersed withinthe elastomeric resin.

Elastomeric Resin

The elastomeric resin can comprise an elastomeric resin suitable for usein an additive manufacturing process. Such materials are well known inthe art. In some examples, the elastomeric resin can comprise athermoplastic polymer such as acrylonitrile butadiene styrene (ABS),polyphenylene sulfide (PPS), polyphenylsulfone (PPSU),polyetheretherketone (PEEK), polyurethane (PU), polyetherimide (PEI),polyphenylene ether (PPE), polycarbonate (PC), and combinations thereof.In some embodiments, the elastomeric resin can comprise a crosslinkablecomposition (e.g., a blend of monomers, oligomers, and/or polymers whichcan be crosslinked during the additive manufacturing process). Dependingon the additive manufacturing process employed, the crosslinkablecomposition can be selected such that crosslinking can be inducedthermally and/or by impinging electromagnetic radiation (e.g., UV and/orvisible light). In certain embodiments, the elastomeric resin cancomprise a crosslinkable silicone composition. For example, theelastomeric resin can comprise (A) a first organosilicon compound havingat least two ethylenically unsaturated moieties per molecule; andoptionally (B) one or more additional organosilicon compounds. Suitablesilicone compositions are known in the art. See, for example, U.S. Pat.No. 10,155,884 to Dow Silicones Corp., U.S. Patent ApplicationPublication No. 2017/0312981 to Wacker Chemie AG, U.S. PatentApplication Publication No. 2018/0370141 to Wacker Chemie AG, U.S.Patent Application Publication No. 2018/0066115 to Wacker Chemie AG,U.S. Patent Application Publication No. 2018/0186076 to Dow CorningCorp., and U.S. Patent Application Publication No. 2019/0100626 toLawrence Livermore National Security LLC, each of which is herebyincorporated by reference in its entirety. Other suitable elastomericresins are described, for example, in U.S. Patent ApplicationPublication No. 20160319150 to Cornell University.

Optionally, the composition may further optionally a non-magneticfiller. The non-magnetic filler may be, for example, an organic filler,an inorganic filler, a ceramic powder, or combinations thereof. Theorganic filler may be a polymer, such as, but not limited to,polystyrene, polyethylene, polypropylene, polysulfone, polyamide,polyimide, polyetheretherketone, etc. The organic filler can also be asmaller molecule either amorphous or crystalline in nature, and can beof in various shapes and sizes. The inorganic filler or ceramic powdercan be any inorganic compounds that are compatible with the curingchemistry. Examples include, but are not limited to, silicon dioxide,titanium dioxide, zirconium dioxide, barium titanate, strontiumtitanate, etc. A mixture of more than one inorganic or organic withinorganic fillers are also suitable.

In embodiments including the non-magnetic filler, the non-magneticfiller can be present as any suitable wt. % of the composition, such asabout 0.01 wt. % to about 90 wt. %, about 1 wt. % to about 80 wt. %,about 5 wt. % to about 80 wt. %, about 10 wt. % to about 80 wt. %, about15 wt. % to about 80 wt. %, about 25 wt. % to about 80 wt. %, about 30wt. % to about 80 wt. %, about 40 wt. % to about 80 wt. %, about 50 wt.% to about 75 wt. %, about 55 wt. % to about 75 wt. %, about 60 wt. % toabout 70 wt. %, alternatively about 0.1 wt. %, 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 18, 20, 25, 30, 35, 40, 45, 50, 55, 60,65, or about 70 wt. % or more.

The non-magnetic filler can have any suitable particle size, e.g., thelongest dimension of the particle, such as the average longestdimension. For example, the non-magnetic filler can have a primaryparticle size of about 5 to about 100, about 10 to about 90, about 20 toabout 80, about 30 to about 70, about 40 to about 60, or about 50,microns, alternatively 5 microns or less, alternatively 100 microns ormore. As used herein, “primary” particle size refers to the actualparticles in their un-conglomerated state, which can optionallyconglomerate to form larger “secondary” particles.

Any of the compositions may optionally and independently furthercomprise additional ingredients or components (“additives”). Examples ofadditional ingredients include, but are not limited to, adhesionpromoters; dyes; pigments; anti-oxidants; initiators for crosslinking;carrier vehicles; heat stabilizers; flame retardants; thixotropicagents; flow control additives; inhibitors; extending and reinforcingfillers; and cross-linking agents. One or more of the additives can bepresent as any suitable wt. % of the composition, such as about 0.1 wt.% to about 15 wt. %, about 0.5 wt. % to about 5 wt. %, or about 0.1 wt.% or less, about 1 wt. %, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, orabout 15 wt. % or more of the composition.

Methods of Use

In some embodiments, the compositions described herein can be used toform cushions that can be used in areas prone to pressure ulcerdevelopment. The cushions can be used in shoes soles/insoles, casts,pads that can be adhered to the bottom of an amputation stump, gloves,mitts, or clothing or mats to detect when a patient is at risk ofdeveloping buttock, sacral, ischial, scapular, or scalp ulceration.

In some embodiments, the compositions described herein can be used toform shoe soles/insoles. The magnetic field strength can then bedetected by a sensor such as wireless triple axis accelerometer andmagnetometer. The sensors can be embedded in a custom- or generic-madeinsole. In some embodiments, the sensor(s) can be incorporated into theshoe soles/insoles. In some other embodiments, the sensor(s) can beplaced near the shoe soles/insoles (i.e., the tongue of the shoe) as tonot break the sensor due to repeated compression of the shoesoles/insoles.

In some embodiments, additional sensors can be present in the such astemperature, moisture, blood flow and blood glucose sensors.

The shoe soles/insoles can detect the displacement and force of theperson walking based on the magnetic field strength generated from themagnetic particles. In some embodiments, the shoe soles/insoles can beused to help predict possible injuries from repeated stress and strainwithin people and potentially professional athletes.

In some embodiments, the composition can be integrated throughout thecushioning of the shoe for measuring impact and shear forces at fivedifferent areas of the foot. In some embodiments, the shoe sole caninclude two sensors located under the toes, two sensors under thefootpads and a sensor beneath the heel. In some embodiments, thesensor(s) are Hall Effect sensors connected to a microcontroller (e.g.,Arduino) and can wirelessly transfer the data in real-time to a mobiledevice. The data can then be used by the medical provider and the userto suggest changes in activity for treatment, prevention and diagnosisof foot ulcers (e.g., diabetic foot ulcers).

In some other embodiments, the composition can have the ability toreconnect soft tissue offering important clinical benefits for patientswith soft tissue disorders. Self-healing hydrogels with pressuresensitivity and stretchability.

The composition is suitable for use in a conventional additivemanufacturing process, such as fused deposition modeling (FDM), fusedfilament fabrication (FFF), fused pellet fabrication, fused particlefabrication, composite filament fabrication (CFF), direct ink writing(DIW), stereolithography (SLA), digital light processing, continuousliquid interface production, selective heat sintering, or selectivelaser sintering.

The compositions described herein can be used to form 3-dimensionalarticle (e.g., cushions, sensors, or structural members). In someembodiments, the anisotropic magnetic particles are aligned and/ororiented within the article. The articles can be formed by any suitablemanufacturing technique; however, in certain embodiments, the articlescan be formed by an additive manufacturing process.

The articles can be interfaced with a magnetometer, such as a HallEffect sensor, configured to interrogate the magnetic field strengthwithin the article. By measuring the magnetic field, strength within thearticle, a force applied to the article can be calculated.

The articles can be interfaced with a magnet (e.g., an electromagnet)configured to apply a magnetic field within the article. By applying amagnetic field of varying strength, the mechanical properties of thearticle (e.g., the Young's modulus of the material) can be varied.

These effects can be anisotropic in nature.

Also provided are methods of forming 3-dimensional articles using thecompositions described herein via additive manufacturing processes. Forexample, provided are methods of forming 3-dimensional articles using a3D printer having an x-y-z gantry robot that comprise extruding acomposition described herein via a nozzle operatively coupled to thex-y-z gantry robot to form an article having a predetermined shape withthe extruded composition. Extrusion of the composition can align and/ororient the anisotropic magnetic particles within the formed article(e.g., by flow of the anisotropic particles through the nozzle).

Also provided are methods of forming 3-dimensional articles thatcomprise:

(i) applying a composition described herein by an independentlycontrollable apparatus in an x, y work plane via at least one printinghead, to an independently spatially controllable baseplate or to ashaped body affixed thereto;

(ii) allowing the composition to cure or solidify to form a layer of acured or partially cured 3-dimensional article;

(iii) displacing the controllable apparatus and/or the shaped3-dimensional article from step (ii) relative to each other in thez-direction far enough such that a next layer can be applied in the x, ywork plane; and

(iv) repeating steps (i)-(iii) until construction of the 3-dimensionalarticle is complete.

In some embodiments, step (ii) can comprise inducing curing of thecomposition thermally and/or by UV or UV-VIS light.

Application of the composition can align and/or orient the anisotropicmagnetic particles (e.g., by flow of the anisotropic particles throughthe printing head).

By way of non-limiting illustration, examples of certain embodiments ofthe present disclosure are given below.

EXAMPLES Example 1: Magnetic Nanorod Fillers for Silicone Direct-WriteInks

Materials and Methods

Nanorods Synthesis and Characterization.

The nanorods were synthesized by electrodeposition of nickel intonanoporous alumina templates. Whatman™ Anodisc™ Filter Membranes of 0.02μm pore size were obtained from Fisher Scientific (Hampton, N.H.);gallium-indium eutectic, nickel(II) chloride hexahydrate, nickel(II)sulfate hexahydrate and boric acid used in the electrolyte solution wereobtained from Sigma Aldrich (St. Louis, Mo.); nickel wire of 1.0 mmdiameter 99.5% metals basis was purchased from VWR (Radnor, Pa.). Thedeposition time was varied to produce nanorods of assorted lengths. Eachnanorod batch (n=50) was imaged with a light microscope and the nanorodlengths were measured using ImageJ (small=1-4 μm; medium=4-8 μm;large=8-12 μm). Transmission electron microscopy (TEM) was used tomeasure the diameters of the synthesized rods to examine their nanoscalefeatures. Energy dispersive x-ray spectroscopy (EDX) was employed toanalyze the elemental composition and to obtain elemental maps of thenanorod samples.

Synthesis and Mechanical Testing of NuSil R40-2181-Nanorod Composites.

Nanorods of various size ranges (1-4 μm and 8-12 μm) were incorporatedinto NuSil R40-2181 (Nusil Technology LLC; Carpinteria, Calif.), and theviscoelastic properties of each formulation were quantified throughcontinuous flow and oscillatory stress sweeps and recovery tests with anAR 2000 Rheometer (TA Instruments; New Castle, Del.). The continuousflow sweeps were performed at a shear rate of 0.02 s⁻¹, and anoscillatory stress from 1 to 10000 Pa was applied at 1 Hz for the stresssweeps. Rheological recovery tests were also performed by the cyclicapplication of 30 seconds of 4 kPa critical stress followed by 5 minutesof zero stress.

NuSil R40-2181 containing 1 wt % nickel nanorods with lengths of 1-4 μm,4-8 μm and 8-12 μm were 3D printed with an EnvisionTEC Bioplotter(EnvisionTEC; Gladbeck, Germany). Printing parameters (e.g., pressureand plotting speed) were adjusted to yield strand diameters ofapproximately 0.5 mm. Lattice structures with heights of 4 mm wereprinted and cured at 160° C. for 4 hours. A 3 mm biopsy punch (MiltexGmbH; Rietheim-Weilheim, Germany) was used to form cylindrical samplesfor uniaxial compression tests. The average compressive elastic modulus(n=6) for each formulation was determined using an RSA III dynamicmechanical analyzer (TA Instruments; New Castle, Del.). The cylindricalsamples were compressed with the dynamic mechanical analyzer (DMA) to70% of their initial height at a rate of 0.005 mm/s, and the strain andnormal stress were both measured. These values were used to calculatethe compressive elastic modulus and reported as mean±standard deviation.

Synthesis and Mechanical Testing of PDMS-Nanorod Composites in MagneticField.

Nanorods of varying lengths (4-12 μm) were incorporated into DowSYLGARD™ 184 Silicone Elastomer Clear (Dow Chemical Co.; Pevely, Mo.) at1 wt % concentration and cured in flat bottom 96 well plates (CorningIncorporated; Corning, N.Y.) at 60° C. for 1 hour. To producevertically-oriented PDMS-nanorod constructs, a cylindrical electromagnetwas placed below the samples during curing and powered to provide amagnetic field at a strength of 15 gauss during the curing process.Similarly, horizontally-oriented PDMS-nanorod cylinders were prepared byapplying a 15 gauss magnetic field with the electromagnet mountedperpendicularly to the samples during curing.

A cylindrical electromagnet (Uxcell, Hong Kong) was mounted onto the RSAIII DMA and connected to a DC power supply. PDMS constructs with 1 wt %nanorods were mounted on top of the electromagnet and compressed at acontrolled strain rate (0.005 mm/s). DMA testing was performed on eachsample (n=3) with the electromagnet powered at 0, 1, 1.5, 3, 6, and 12V. These studies were conducted on PDMS-nanorod constructs with eithervertically-oriented nanorods or horizontally-oriented nanorods. Themagnetic field strengths produced by the electromagnet at eachcorresponding voltage were measured with SS49E linear Hall effectsensors (Honeywell; Golden Valley, Minn.) controlled by an Arduino Mega2560 microcontroller (Arduino; Somerville, Mass.). Compressive moduli(G′) data obtained from DMA testing were plotted against magnetic fieldstrength values.

Sectioning and Imaging of PDMS-Nanorod Cylinders.

PDMS constructs with horizontally-oriented and with vertically-orientednanorods at a concentration of 1 wt % were suspended in optimal cuttingtemperature (OCT) compound (Scigen Scientific; Gardena, Calif.) andsubsequently frozen at −80° C. overnight. A cryostat (Leica Biosystems;Wetzler, Germany) was used to cut cross sections of the PDMS-nanorodcylinders at a thickness of 50 μm. Cross sections of PDMS withhorizontally-oriented and vertically-oriented nanorods were then imagedwith a bright-field microscope with a 40× objective lens.

Remote Sensing of Compressive Forces Applied to PDMS-Nanorod Constructs.

An MLX90393 triple-axis magnetometer breakout board (SparkFun; Boulder,Colo.) was wired to an Arduino Mega 2560 microcontroller to communicatevia I2C, and the magnetometer board was then mounted onto the bottomgeometry platform of the RSA III DMA. PDMS-nanorod constructs with 1 wt% nanorods were subsequently placed above the magnetometer sensor, andthe DMA was utilized to exert increasing increments (100-600 grams in100 gram increments) of compressive force on the PDMS-nanorodconstructs. Magnetic field strength data from all 3 axes were collectedfrom the output of the Arduino IDE software while the samples werecompressed by the DMA. PDMS-nanorod samples were compressed from 100-500grams of force for 5 seconds each up to 500 grams of force and for 80seconds at 600 grams of force. Magnetic field strength readings from themagnetometer were plotted against time. Remote sensing of compressionevents was successfully achieved; both duration and magnitude ofcompressive forces were detectable based on the change in magnetic fieldstrength. Additionally, the aforementioned procedure was conducted withblank PDMS constructs without nanorods to confirm the validity of theresults (i.e., lack of interfering external forces) and techniqueoutlined above.

Statistical Analysis.

All results were presented as the mean±standard deviation (SD).Statistical analysis was performed by SAS® 9.4 Software using one-wayanalysis of variance (ANOVA) in conjunction with Tukey's HSD post-hoctest to compare between individual groups. Statistical significance wasdefined by p-values of *p<0.05, **p<0.01, and ***p<0.001.

Results

Nanorods Characterization.

The average nanorod length per batch (n=50) as a function of depositiontime is shown in FIG. 1. Based on the TEM images that were obtained(FIGS. 2A-B), rods synthesized by the template-based method possessedconsistent diameters on the nanoscale ranging from 130-300 nm. Elementalmaps of the nanorod samples were obtained using energy dispersive X-rayspectroscopy (FIGS. 2C-2H). Elemental analysis confirmed that thenanorods were primarily composed of nickel elements (82.42% by weight),and that the nickel elements were evenly distributed throughout.

NuSil R40-2181-Nanorod Composites.

Medium (4-8 μm) and large (8-12 μm) size nickel nanorods wereincorporated into NuSil R40-2181 and the viscoelastic properties of eachformulation were quantified through rheology. Representative continuousflow and oscillatory stress sweep curves are shown in FIGS. 3A-3B. Eachformulation exhibited shear thinning behavior and had an observableyield stress (Table 1). Overall, the addition of 1 wt % medium rods and1 wt % large rods did not significantly impact the apparent viscosity,storage modulus, loss modulus and yield stress of NuSil R40-2181. Therheological recovery experiments (FIG. 3C) showed that the materialsexhibited only partial recovery within the 5-minute timeframe followingoscillatory disruption. After four cycles of applied stress, the storagemoduli for NuSil R40-2181 containing 0%, 1% medium rods and 1% largerods were only recovered by 49%, 43% and 39%, respectively. The datasuggest that the materials are deformed by shear stresses of criticalmagnitudes, and that the addition of nanorods impairs the recovery ofbulk fluid properties, although the effect is marginal. Latticestructures of NuSil R40-2181 containing 1 wt % nickel nanorods of small(1-4 μm), medium (4-8 μm) and large (8-12 μm) lengths were 3D printedwith strand diameters of ˜0.5 mm. Two-layered lattice structures wereprinted and flow-induced nanorod alignment was assessed and confirmed tosuccessfully dictate the orientation of the nanorods (FIG. 4).

Dynamic mechanical analysis (DMA) indicated that the addition of smallrods did not significantly affect sample stiffness, whereas the additionof medium and large rods resulted in a statistically significant(p-value <0.0001) increase in stiffness (FIG. 5A). These resultsindicate that the length of loaded nanorods influences the stiffness of3D-printed NuSil R40-2181. The inclusion of nickel nanorods of alllengths at a 1 wt % concentration also endowed the material system withobservable magnetism, as the 3D-printed structures were attracted to anexternal magnetic field (Table 1). Since nanorod alignment is inducedthrough the shear and extensional stresses applied during the 3Dprinting process, the mechanical tests were performed on 3D-printedsamples containing nanorods predominantly aligned in the horizontaldirection.

TABLE 1 Rheological, mechanical and magnetic properties ofnanorod-loaded NuSil R40-2181. Compressive Observable Material YieldStress Modulus Magnetic Properties Formulation Pa kPa Y/N 0% Rods 1552 ±3  795 ± 106 N 1% Small Rods TBD  744 ± 42 Y 1% Medium Rods 1548 ± 51080 ± 74 Y 1% Large Rods  1378 ± 155 1240 ± 80 Y

To test the effect of nanorod orientation on mechanical stiffness,cylindrical samples were punched out of the side of the 3D printedlattice structures as opposed to the top of the structures, to ensurethat nanorods were aligned in the vertical direction every alternatelayer. The compressive moduli of “horizontal” and “vertical” orientedNuSil R40-2181 samples without nanorods were not significantlydifferent. However, “vertical” NuSil R40-2181 samples loaded with 1 wt %medium sized nanorods exhibited statistically significant (p-value<0.0001) larger moduli than “horizontal” samples (FIG. 5B),demonstrating that the compressive stiffness of 3D-printed structures isenhanced by nanorods that are vertically aligned.

PDMS-Nanorod Composites.

Small (1-4 μm) and large (8-12 μm) size nickel nanorods wereincorporated into SYLGARD™ 184 Silicone Elastomer Clear (PDMS) at 1 wt %concentration and observable magnetism was achieved. The nanorods wereoriented vertically and horizontally within the constructs, and thisorientation was confirmed using brightfield microscopy on cross sectionsof the cured PDMS-nanorod composites (FIGS. 6A-6B). Thehorizontally-oriented nanorods look like long rods throughout the crosssection, perpendicular to the objective lens. The vertically-orientednanorods appear as dots indicative of the rods' diameters, demonstratingthat the rods were successfully oriented in the same direction as thelens.

Compression DMA studies showed that the addition of large nanorodsresulted in a higher compressive modulus than that obtained through theaddition of small nanorods (FIG. 7A). When the length of the rods wereincreased in the PDMS samples, a higher Young's modulus was obtained.This result ratifies the direct proportionality of nanorod size tosample stiffness observed in the NuSil R40-nanorod composites. Nanorodlength strongly dictated the mechanical properties of both siliconecomposites.

Exposure to a Magnetic Field.

The PDMS-nanorod composites were exposed to a magnetic field underneathduring compression DMA studies to study the mechanical properties inrelation to the strength of a magnetic field applied. FIG. 7A shows thatfor vertically oriented nanorod composites, the application of amagnetic field did not yield a significant change in compressive modulusregardless of the size of the nanorods. The large rods provided stiffercomposites than the small rods regardless of the strength of themagnetic field applied. However, this was not the case for thehorizontally oriented nanorod composites (FIG. 7B). When exposed to amagnetic field, the horizontally oriented nanorod composites experiencedlarge increases in compressive modulus. This increase in compressivemodulus was related to the strength of the magnetic field applied,increasing rapidly when exposed to weaker magnetic fields but not asrapidly for stronger magnetic fields. When no magnetic field wasapplied, the 1 wt % vertically oriented nanorod composites displayed ahigher compressive modulus than that of the horizontally orientednanorod composites. Applying a magnetic field, however, enhances thecompressive modulus of the horizontally oriented nanorod composites,while the vertically oriented nanorod composites remained constant. PDMSsamples with no nanorods were also compressed dynamically while beingexposed to a magnetic field and no change in compressive modulus wasobserved, confirming that the increase in stiffness is caused by theactuation of the nanorods within the composites and not by the PDMSitself.

The orientation of the nanorods provides the user with the ability tomodulate the stiffness of the material by applying a magnetic field.Horizontally oriented nanorod composites can be used when enhancedmechanical strength is needed on an application, and this can be tunedwith the strength of the magnetic field applied. Conversely, if amagnetic silicone construct that maintains its mechanical propertiesregardless of the magnetic field applied is needed, the verticallyoriented nanorod composites are optimum given that their stiffness hasvery little to no response to magnetic field strength.

Detection of Compressions with Magnetometer.

Using a triple-axis magnetometer (Hall Effect sensor) and Arduinomicrocontroller, PDMS-nanorod (1 wt % nanorods) composites weresubjected to compression and the change in magnetic field strength wasmeasured to determine if mechanical forces can be remotely sensed.Increasing compressive forces in increments of 100 g were applied, andthe fluctuation in magnetic field strength varied at each increase incompressive force. Not only was the magnitude of force remotelydetectable by the magnetometer, but also the duration of the forceapplied (FIG. 8A). After compressing the PDMS-nanorod constructs at 600g for 80 seconds, the compressive force was removed, and the samplereturned from a field strength of approximately −400 μT to the baselinemagnetic field strength reading (approximately −800 μT). These resultsconfirm the ability to remotely sense compression events and othermechanical forces in PDMS-nanorod structures with detection of bothduration and magnitude. Furthermore, the data confirmed thatPDMS-nanorod constructs retained their initial magnetic field strengthsafter cycles of compression. Finally, the 3D printed silicone cushionscontaining 1 wt % nanorods (see FIG. 4) were also compressed withvarying forces. A similar compression cycle was applied and the siliconecushions demonstrated similar changes in magnetic field strength (FIG.8B) validating remote sensing of these soft materials.

Findings indicated that the orientation of the nickel nanorods in thecomposite affected the magnetic field strength. At the same compressionintervals, PDMS cylinders with horizontally aligned nickel nanorodsdisplayed a greater increase in magnetic field strength when compared tocylinders with vertically aligned nickel nanorods (FIG. 10).

Sensing of movement, especially that in three directions, is animportant characteristic of the next generation of smart materials. Inevaluations of the PDMS-nickel nanorod 1 wt % composite the cylindermovement was sensed in the three directions and displacement ofdifferent magnitudes and durations were evident (FIG. 11).

The results from these investigations by our colleagues were thatsilicone cushions with a PDMS-nickel nanorod 1 wt % compositiondisplayed electromagnetic-induced stiffness with remote control and thatmovement in a three-directional fashion was sensed using a remotemagnetometer. The findings also suggest that horizontally aligned nickelnanorods display a greater change in magnetic field strength thanvertically aligned nanorods.

Example 2: Compression Testing of Ferromagnetic Particles within SoftMaterials

The inclusion of magnetic particles as fillers within soft materials hasthe potential to drive the development of smart materials with highfunctionality and structural diversity. Six ferromagnetic fillers (i.e.,nickel, carbonyl iron, cobalt, iron oxide, magnetite, and neodymiumpowder) were incorporated within polydimethylsiloxane at concentrationsof 0.01 wt %, 0.1 wt %, and 1 wt %. Defined compression tests determinedthe ability to detect material deformation and the magnetic fieldresponse generated during compression cycles. Utilizing iron oxide at 1wt %, the compressive response of additional silicones and a two-partpolyurethane was also investigated.

Compression testing of five of the six ferromagnetic fillers in PDMS,with the exception of carbonyl iron, revealed that 1 wt % was theminimum concentration required to detect compression events via themagnetic field response. The findings of carbonyl iron at 1 wt % werenot viable as its magnetic field response was similar to that of thePDMS control samples. The neodymium filler particles produced thestrongest magnetic field response. However, settling of the neodymiumparticles became evident during the curing process, which promptedfurther theoretical exploration at various particle sizes andviscosities. PDMS displayed the optimal relationship between force anddisplacement amongst the various polymers with 1 wt % iron oxide. Theother materials were either too soft or were too resistive to beconsidered viable as a durable soft sensor material or were limited byan inability to measure magnetic field strength.

Introduction

The rapid advancements in soft materials has spurred the development ofsmart sensing devices including those that may be implantable intovarious textiles or other sensing applications. These smart materialsare comprised of magnetic particles to enable detection of materialmovement or even control of movement via external magnetic fields. Suchsoft materials can be applied in a number of fields which include softrobotics^(22, 23), biomechanics^(24, 25), as well as andbiomedical^(26, 27) and biotechnology²⁸ applications such as drugdelivery²⁹, imaging^(30, 31) monitoring^(14, 32,) and invasivesurgery^(33, 34) devices. However, there is still a need to improve theaccuracy and performance of these soft sensors.

Until recently, Hall effect sensors generally utilized a solid and rigidneodymium magnet embedded within silicone or other soft materials. Thesewere limited both by its structure but also because these kinds ofsensors tend to reach saturation quickly and therefore offer a limitedrange of measurement¹⁰. The need to improve these sensors lead to aninvestigation designed to identify a way to replace rigid neodymiummagnets with a neodymium magnetic powder that could then be blended withsilicone and cured¹¹. This technique reduced the overall thickness andresulted in the development of a sensor with a softer structure.

Numerous synthetic polymers and soft materials are on the market,however the need to improve materials and performance of soft sensors isfundamental³⁵. Soft materials need to be durable as to not break fromrepeated amounts of compression and have sufficient recovery to sustaintheir shape. They also need to maintain the ideal rheological propertiesprior to curing if additive manufacturing techniques of these softsensors is to be explored^(36, 37). Silicones have become favorable dueto their low cost as well as their ability to be printed through directink writing (DIW)³⁸. They also express the ability to change physicalproperties due to varying molecular weight, chemistry, andweight-to-weight (w/w) ratios of the base polymer tocrosslinker^(39, 40).

Magnetic particles used as fillers for soft materials would need topossess the ability to emit a large magnetic field and retain theirmagnetic moment. In addition, they would have a high remnantmagnetization and have a large coercivity to avoid demagnetization.Before chemical synthesis of ferromagnets, this was considered ananomaly because materials that have a large coercivity tend to have lowremnant magnetization and those with a high remnant magnetization tendto have a low coercivity³⁰. Therefore, we explored syntheticferromagnetic fillers such as cobalt, iron, nickel, and their commonalloys because of their abilities to retain permanent magnetism. We alsoconsidered the particle shape and sizes of these magnetic fillers in aneffort to avoid affecting the bulk properties of the soft materials.This is especially vital in situations where the function of the softmaterial must be maintained.

The current state of the art in magneto sensing has revealed severallimitations with regards to both magnetic particle performances as wellas materials. Specifically, the key components are to improve uponsensitivity, accuracy, and performance^(9, 10). This study sought toadvance previous magnetic sensing research and our own collaborativedata to improve current force sensors, specifically the limitations insensitivity and accuracy. We designed these investigations to identifyand test six ferromagnetic fillers incorporated within soft sensors. Ourmain objective was to analyze the magnetic concentration dependence onthe ability to detect compression events and their magnetic fieldresponse generated during these compression events. Next, we wanted toassess a particular ferromagnetic filler with a viable magnetic fieldresponse to identify other possible soft materials and composites withpotential in future sensing applications.

Materials & Methods

Nickel Nanorod Synthesis

Ferromagnetic nickel nanorods utilized in the testing were synthesizedby electrodeposition of nickel into nanoporous alumina templates⁴¹. Toperform the process supplies were acquired as follows: Whatman™ Anodisc™Filter Membranes of 0.02 μm pore size were obtained from FisherScientific (Hampton, N.H., USA); gallium-indium eutectic, nickel(II)chloride hexahydrate, nickel(II) sulfate hexahydrate and boric acid usedin the electrolyte solution were obtained from Sigma Aldrich (St. Louis,Mo., USA); and nickel wire of 1.0 mm diameter 99.5% metals basis waspurchased from VWR (Radnor, Pa., USA). Previous work demonstrated thatvarying the deposition time produced nanorods of assorted lengths: 10-20minutes produced small (1-4 μm), 20-30 minutes produced medium (4-8 μm),and 40-50 minutes produced large (8-12 μm) nanorods. All nickel nanorodssynthesized and produced for this work utilized a deposition time of 45minutes.

Remaining Magnetic Materials

Aside from the nickel nanorods being produced in lab, the five remainingferromagnetic materials were obtained from and used as received fromoutside suppliers. Carbonyl iron microspheres, cobalt nanowires, andiron(II,III) oxide (iron oxide) nanopowder were all acquired from SigmaAldrich (St. Louis, Mo., USA). The iron oxide contains a 97% trace metalbasis. Magnetite powder and neodymium iron boron (neodymium) powder werepurchased from Advanced Reade Materials (East Providence, R.I., USA) andNanoshel (Wilmington, Del., USA), respectively. The purity of theneodymium powder is 95-96% and comprised of 29-32% neodymium, 64.2-68.5%iron, 1.0-1.2% boron, and 0.5-1.0% niobium. Physical properties of themagnetic materials were provided in their technical data sheets by theirrespective suppliers and are summarized in Table 1.

TABLE 1 Physical properties of ferromagnetic materials. Diameter/AverageMagnetic Material Particle Size Length (μm) Density (g/cm³) CarbonylIron 1 μm — 7.86 Cobalt 200-300 nm 100-200 8.90 Iron Oxide 50-100 nm —4.8-5.1 Magnetite 5 μm — 5.1  Nickel 100-200 nm  8-12 — Neodymium 50-60μm — 7.5 

Characterization

Each magnetic material was imaged using a light microscope and theirparticle sizes and lengths were verified using ImageJ. Previous workutilized transmission electron microscopy (TEM) to measure the diametersof the synthesized nickel nanorods and to examine their nanoscalefeatures.

Soft Materials

Three silicones and a two-part polyurethane were also purchased fromoutside suppliers to be utilized as soft materials. Dow Sylgard™ 184silicone elastomer base and curing agent was acquired from Dow ChemicalCompany (Pevely, Mo., USA). It is a two-part system comprised of apolymeric base and a curing agent that when combined, the curing agentcrosslinks with the polymeric matrix to form PDMS. Two additional vinylterminated polydimethylsiloxanes (DMS-V21 and DMS-V33) andplatinum-cyclovinylmethyl-siloxane complex; 2% pt incyclomethylvinylsiloxanes (platinum catalyst) were purchased from Gelest(Morrisville, Pa., USA). The Sylgard 184 curing agent was used as thecrosslinker and the platinum catalyst was also used to help cure thesetwo siloxanes. The supplier recommends using a 10:1 w/w ratio of base tocuring agent for PDMS, however, a 15:1 w/w of base to curing agent wasused for all three silicones. A two-part polyurethane (BJB) was alsoobtained from BJB Enterprises (Tustin, Calif., USA). The polyurethanemixture needs equal parts (1:1 w/w) of each component to cure.

Table 2 lists the respective densities (kg/m³) and viscosities (cSt) ofthe soft materials used as provided by their respective suppliers'technical data sheets.

TABLE 2 Physical properties of uncured soft materials. Soft MaterialDensity (kg/m³) Viscosity (cSt) BJB 1,050 1,100 DMS-V21 Base 970 100DMS-V33 Base 970 3,500 PDMS Base 1,110 5,000 Sylgard 184 Curing Agent1030 110

Preparation of Samples

The dry powder of all six ferromagnetic materials were incorporated intoPDMS at varying concentrations of 0.01, 0.1, and 1 weight percent (wt%). The powder was first weighed before adding the PDMS base and curingagent at a 15:1 w/w ratio. The samples were hand mixed aggressivelyusing glass stirring rods and then centrifuged at 500 min′ to eliminateany air bubbles without separating components. Uncured samples were thenmixed lightly again before being filled into 96 well-flat bottom plates(Corning Incorporated; Corning, N.Y., USA) and placed in a vacuumchamber for 20 to 30 minutes before curing to remove any additional airbubbles (FIG. 14A). Following this process, the samples were then placedin an oven at 70° C. to decrease curing time from 24-48 hrs at roomtemperature to just 2-3 hours. To produce horizontally-oriented dipolesof ferromagnetic particles within the PDMS, samples were prepared byapplying a 15 gauss magnetic field (˜0.1 T) with an electromagnetmounted perpendicularly to the samples during curing (FIG. 14B).

After curing was completed, the well plates were broken down and sampleswere removed from individual wells using ethanol. Due to the siliconessurface tension within the individual wells, menisci had formed and wereremoved by slicing the top part of the cylindrical constructs to bringtheir height down from about 10.65 mm to approximately 7.50 mm. Sampleswere also brought down to this height to help prevent shear stressduring testing. The samples each had a diameter of 6.5 mm.

Iron oxide nanopowder was also mixed with the remaining silicones,DMS-V21 and DMS-V33, as well as the BJB polyurethane at 1 wt %. The ironoxide was utilized as the control within these soft materials due to itsabundance and inexpensive cost as well as its magnetic field strengthdisplayed in preliminary studies. The DMS-V21 and DMS-V33 were preparedin similar fashion to the PDMS as each was used at a 15:1 w/w ratio ofbase to curing agent with an additional 0.01 volume % of platinumcatalyst. with the iron oxide following the weighing, centrifuging,vacuum chamber, and oven. The BJB polyurethane cures within 30 minutesat room temperature so once it was mixed and centrifuged with iron oxideat 1 wt %, it was left to cure at room temperature.

Compression Testing and Sensing

A MLX90393 triple-axis magnetometer purchased from Adafruit Industries(New York City, N.Y., USA) was wired to an Arduino Mega 2560microcontroller to communicate via I2C, and the magnetometer board wasthen mounted using double sided tape onto the bottom geometry platformof the Electroforce 5500 (TA Instruments, Eden Prairie, Minn., USA).Cylindrical constructs were subsequently placed above the magnetometersensor and the ElectroForce 5500's axial mover was lowered until contactwas made with the top side of the sample (FIG. 14A). The ElectroForcewas utilized to exert increasing increments (i.e., 0-600 g in incrementsof 200 g over 10 seconds) of compressive force on the samples (FIG. 14Band C) for the first half of the experiment and then release those sameincremental amounts of force until zero force is applied at the end.Each compressive force was held for 10 seconds. Magnetic field strengthdata from all three axes were collected from the output of the ArduinoIDE software while the samples were compressed by the ElectroForce 5500.Magnetic field strength readings from the magnetometer were plottedagainst time alongside the applied force. Remote sensing of compressionevents was successfully achieved; both duration and magnitude ofcompressive forces were detectable based on the change in magnetic fieldstrength. This procedure was also conducted with blank PDMS, DMS-V21,DMS-V33, and BJB constructs without the ferromagnetic materials toestablish a baseline reading of magnetic field strength and possiblebackground interference.

Prior to using the ElectroForce 5500, preliminary studies were alsoperformed using a similar technique as seen in FIG. 14A-C. However,rather than measuring the magnetic field response based on appliedforce, the magnetic field strength was measured against displacement. ANEMA-23 two-phase stepper motor was modified with platforms to addcompression geometry onto the linear actuator. This custom built steppermotor seen in FIG. 15A was then used to apply increasing increments(i.e., 0-2.25 mm in increments of 0.75 mm) of displacement on thesamples seen similarly in FIGS. 14B and C. A LSM303 triple-axisaccelerometer and magnetometer purchased from Adafruit Industries wasmounted onto the bottom platform geometry using double sided tape. Thecylindrical construct was then placed on top of the magnetometer beforethe top geometry platform (mounted to linear actuator) was lowered untilcontact with the top side of the sample was made (FIG. 15B). Twoseparate Arduino Mega 2560 microcontrollers were then used tocommunicate with the magnetometer and the stepper motor via an A4988stepper motor driver.

Experiments can also be performed similar to the force and displacementtests that were explained by the ElectroForce, but rather than placingthe triple axis accelerometer and magnetometer directly between thebottom of the cylindrical samples and the bottom of the ElectroForcegeometry, a structure to support the magnetometer directly perpendicularto the sample and the ElectroForce can be implemented. In the currentassessment, the cross-sectional area of the actual sensor on themagnetometer board is much smaller than the cross-sectional area of thecylinders. As a result, the outcomes may have been affected by thecylindrical constructs beginning to engulf the sensor itself as theElectroForce continuously displaced the sample.

Results & Discussion

Cylindrical constructs consisted of the six different magnetic fillersmixed with PDMS at varying concentration levels of 0.01, 0.1, and 1 wt %to determine the sensitivity for analyzing magnetic field strength (μT).These six ferromagnetic particles consisted of nickel nanorods, carbonyliron microspheres, cobalt nanowires, iron oxide nanopowder, magnetitenanopowder, and neodymium iron boron powder. Iron oxide was also mixedwith DMS-V21, DMS-V33, and BJB at 1 wt %. Blank cylinders of BJB,DMS-V21, DMS-V33, and PDMS with no magnetic nanoparticles served ascontrols to provide a baseline reading on the magnetometer and identifyany possible background interference.

Before these cylindrical constructs were produced, the average particlesizes of these six ferromagnetic materials were verified under a 20×light microscope. FIG. 16A displays the nickel nanorods and the carbonyliron, cobalt, iron oxide, magnetite, and neodymium particles can be seenin FIGS. 16B, C, D, E, and F, respectively.

Samples were made in batches and three constructs of each particle wereselected for testing (n=3). Overall, 72 samples were tested and theensuing results of the magnetic field strength and displacement (mm)were averaged. Criteria for testing included: 1) no visible air bubblesalthough this standard became harder to implement in samples at 1 wt %due to their opaqueness as well as within BJB since it cured so quicklyand 2) most perpendicular top sides after samples were sliced down from10.65 mm to 7.5 mm in height to help minimize possible shear stress. Itshould also be noted that during preliminary trials, only one (n=1) ofevery sample made from nickel, carbonyl iron, iron oxide, and magnetiteat 0.01, 0.1, and 1 wt % as well as cobalt at 0.01 and 0.1 wt % withinPDMS were tested.

Cylindrical constructs of every magnetic material at 0.01, 0.1, and 1 wt% within PDMS was photographed to show how each cured sample appearedafter being cured, removed from well plates and sliced down from about10.65 mm to approximately 7.50 mm (FIG. 17).

FIG. 18A displays the blank samples of BJB, DMS-V21, DMS-V33, and PDMSwith no magnetic filler while FIG. 18B presents each of these softmaterials with 1 wt % iron oxide. The blank DMS-V33 sample appeareddifferent from the rest of the blanks due to the platinum catalyst thatwas added. DMS-V21 also includes a platinum catalyst but did not displaythe darker coloring.

The magnetic field response on the z-axis of each sample was plottedagainst time along with the change in applied force against time todetail the dependence of the magnetic field response based on force.FIG. 19 showcases a typical graph of the magnetic field strength andforce plotted against time for a sample of nickel at 1 wt %. The datawas normalized to display the magnetic field response, which was about1.5±1.5 μT when 0+10 g of force was applied. As the force increased to200±10 g, the magnetic field strength increased. When the force was heldat a certain point, the magnetic field strength would hold as well.Then, as force was unloaded incrementally, the magnetic field strengthof the sample would adjust back to a similar response as when the sameforce was applied in the first half of the test. This is evident in FIG.19 when the sample is held at 400±10 g of force from 40-50 s anddisplays a magnetic field response of 10.7±1.5 μT. Then as the force isincreased to 600±10 g and reduced back to 400±10 g at 90-100 s, thesample exhibits a magnetic field response of about 11.6±1.5 μT.

FIG. 20 outlines the varying ferromagnetic fillers in increasing orderof magnetic field strength when 0, 200, 400, and 600 g of force wasapplied. Neodymium provided the greatest response in magnetic fieldstrength during compression. At 1 wt % and with 600 g of applied force,neodymium exhibited a magnetic field response of 35 μT. Even with only200 g of force applied by the ElectroForce, the neodymium cylindersstill provided a magnetic field response (16 μT) equivalent as the nextstrongest material, nickel, at their maximum force of 600 g. While thenickel was expected to provide a strong magnetic field response, thecobalt was originally hypothesized to produce an even stronger response.However, the cobalt may have displayed a limited magnetic response dueto aggregation of the nanowires observed in the cylinders. The blankPDMS control sample provided a baseline reading of about 1.7 μT ofbackground interference on the magnetometer at 0 g of force and about2.3 μT at about 600 g of force. This could potentially be due to the topgeometry and the axial mover interfering with the magnetometer as itapproached the magnetometer during compression steps. The iron oxide andmagnetite displayed a magnetic field response of about 5 and 4 μT,respectively, at 600 g of force. Meanwhile, the carbonyl iron did notdisplay a magnetic field response strong enough for us to be confidentof detection as its maximum response was about 2.5 μT at 600 g of force.

The ElectroForce was also used to determine the displacement of thesamples by measuring the displacement of the axial mover from itsstarting position to each step of compression. The main motive forquantifying displacement was to identify a similarity betweenpreliminary results performed on the stepper motor compared to theresults from the ElectroForce. FIG. 21 provides the displacement at ofthe same 1 wt % nickel sample seen in FIG. 19. During 600±10 g of forceat 60-80 s, the sample provided a response of about 16.5±1.5 μT as itwas being displaced about 1.83±0.02 mm.

The average displacements of the three samples tested for each materialat 1 wt % during each step of compression can be seen in FIG. 22.Although we originally hypothesized each magnetic filler material wouldhave increased the compressive modulus of PDMS, we can see that cobalt,nickel, and neodymium were all displaced an average of 1.82, 1.78, and1.84 mm, respectively compared to the blank PDMS samples only beingdisplaced 1.62 mm at 600 g of force. Alternatively, carbonyl iron,magnetite, and iron oxide all displayed a higher compressive strengthwith a lower average displacement of 1.14, 1.44, and 1.46 mm,respectively.

Returning to the 1 wt % nickel sample seen in FIG. 21, FIG. 23 providesthe actual displacement as a function of force and display a near linearrelationship. The compression and decompression cycles are very similar,yet distinct due to the recovery of the constructs. After the forceversus displacement curves were plotted for every sample at every wt %,the correlation was used to determine the amount of force applied fromthe stepper motor at each step of displacement. FIG. 24 displays thecompressive force vs displacement for one sample of each magneticmaterial at 1 wt % within PDMS. With this information, a relationshipbetween displacement and magnetic field strength could be modelled infuture investigations.

When implementing these soft sensors in other applications, we wouldlike to correlate the force and displacement as a function of themagnetic field strength generated during the unknown amount of force anddisplacement applied. FIG. 25 depicts the relationship betweendisplacement and magnetic field response. For example, with neodymiumpowder incorporated into PDMS at 1 wt %, if the sensor displayed aresponse of about 30 μT, we could determine the sensor was displacedabout 1.4 mm. We can also determine that about 400 g of force wasapplied during that response when using that information and referringback to FIG. 24.

FIG. 26 displays the magnetic field strength produced from the z-axis ofa 1 wt % nickel sample originally tested during the preliminary studieson the stepper motor. The sample produced an average magnetic fieldresponse of 11.0, 19.3, and 20.2 μT at 0.75, 1.5, and 2.25 mm ofdisplacement, respectively. Using the linear equation in FIG. 23 as wellas the linear relationships produced from the other two 1 wt % nickelsamples, we were able to determine approximately how much force wasapplied at each step. At 0.75 mm of displacement, roughly 190 g of forcewas applied while about 450 g and 700 g of force were applied at 1.5 mmand 2.25 mm steps of displacement, respectively.

During our initial evaluations, we were able to identify the magneticfield strength of nickel, carbonyl iron, iron oxide, and magnetite at 1wt % within PDMS when displaced a total of 2.25 mm. Nickel provided avery strong magnetic field response of 20.25 μT (FIG. 27). Iron oxideand magnetite also displayed viable responses of 9.32 and 5.35 μT,respectively. A preliminary objective of this study was to deliver asensitivity analysis of the lowest concentration (wt %) possible toobserve a change in magnetic field strength from these ferromagneticmaterials. From the initial studies depicted in FIG. 27, we can see that1 wt % carbonyl iron only provided a response of 1.08 μT when displaced2.25 mm by the stepper motor whereas, as illustrated in FIG. 20, it onlydisplayed a response of 2.46 μT when displaced 1.14 mm by 600 g of forcefrom the ElectroForce. Both of these responses were relatively close tothe blank PDMS cylinders which displayed a magnetic field response of0.61 μT in FIG. 27 and 2.27 μT in FIG. 20. This helped us determine thatcarbonyl iron is not providing a reliable magnetic field response whencompressed and these values were likely background interference.

Similarities between the iron oxide and magnetite were evident from theinitial evaluations (FIG. 27). Based on the results depicted in FIG. 20and FIG. 22, we can see that magnetite displayed a magnetic fieldresponse of 4.11 μT when it was displaced 1.44 mm by 600 g of force. Theiron oxide also provided a magnetic field response of 5.33 μT whendisplaced 1.45 mm by the same amount of force. If we look at the 1.50 mmbar in FIG. 27, we can see magnetite and iron oxide produced magneticfield responses of 3.17 and 7.58 μT, respectively, which is consistentwith the results seen with the ElectroForce.

As seen from FIG. 28, the lowest wt % possible for detection of theseferromagnetic fillers occurred at 1 wt % concentration for this study.At 600 g of force, neodymium and nickel only displayed readings of 2.80and 2.22 μT at 0.1 wt %, respectively, compared to PDMS which displayeda magnetic response of 2.27 μT at 600 g of force. The remaining magneticmaterials also all displayed responses below 3 μT at 0.1 wt %. This wasnot a reliable response for detecting magnetic field strength withinconstructs at 0.1 wt % or lower at 0.01 wt %.

The displacements of each magnetic filler material at each wt % wasobserved when 600 g of compressive force was applied to determinecompressive strengths within PDMS. We originally hypothesized that themagnetic materials would increase the compressive modulus of PDMS,therefore making each sample more resistive to displacement as fillercontent increased. However, as seen in FIG. 29, we can see there areinconsistent trends of displacement between separate magnetic materialsat varying wt %'s. There are a few possible causes for such altereddisplacements between samples. Most notably, if the w/w ratio ofsilicone base to curing agent is altered slightly from 15:1, this wouldaffect material stiffness. If the ratio is lowered, the silicone willdevelop a higher compressive strength and be more resistive todisplacement. On the other hand, if the ratio is increased, the curedsilicone will have a lower compressive strength and be displaced more⁴⁰.

Additional studies were conducted to explore a magnetic filler materialwithin other soft materials. Due to its abundance and relatively lowcost, iron oxide was selected as the ferromagnetic compound to be mixedwith BJB, DMS-V21, and DMS-V33. FIG. 30 displays the magnetic fieldresponse from the blank soft materials with no iron oxide to establish abaseline as well as the samples with 1 wt % iron oxide. PDMS showed thehighest response in magnetic field with 1 wt % iron oxide with anaverage reading of 5.33 μT at 600 g of compressive force. DMS-V21presented the second highest response of 4.35 μT while DMS-V33 and BJBdisplayed a response of 3.41 and 2.41 μT at 600 g of force,respectively. Notably, BJB showed no promise as a soft material, butthis is because the material did not compress at 600 g of force. FIG. 31shows that the BJB-Blank and BJB-iron oxide samples were only displacedan average of 0.58 and 0.41 mm, respectively. Although the DMS-V21 andDMS-V33 showed some promise, it should be noted that the blank samplesof these silicones displayed higher average magnetic readings than thePDMS and BJB. This could be due to a greater magnetic backgroundinterference from the ElectroForce's top geometry for these samples. Theaverage displacement of the blank DMS-V21 samples were more than twiceas large as the DMS-V33 and PDMS blanks. With the displacement of theDMS-V21 and DMS-V33 samples being larger than that of the PDMS, it wouldhave also been expected that the magnetic field response of thesesilicones would also have been larger than PDMS when iron oxide at 1 wt% was incorporated. However, this was not the case as the PDMS-ironoxide still showed the greatest magnetic field response. Therefore, itis hypothesized that the platinum catalyst may have played a role inaltering magnetic strength readings within the DMS-V21 and DMS-V33.

Although DMS-V21 also seemed that it would show some promise as apotential material for soft sensors, when we view FIG. 32, we can seethat it does not have much compressive strength. In fact, compressioncycles were originally intended to go up to 750 g of force, but DMS-V21samples would fail around 700-750 g of force.

Settling velocities of magnetic particles was also investigated, sincesettling became evident during mixing of neodymium within PDMS at 1 wt%. In order to calculate settling time and viscosities, densities andviscosities of the siloxanes and their curing agent at specificweight-to-weight ratios were required. Mixture density, ρ, wascalculated as the summation of the mass fraction, x, multiplied by thedensity of each component as seen in Equation 1⁴².ρ_(n)=Σ(x _(i)*ρ_(i))_(n)  (1)

Equation 2 was used to calculate the natural log of the mixtureviscosity, □, as the natural log of each component's viscositymultiplied by its mass fraction⁴³.ln η=x ₁ ln η₁ +x ₂ ln η₂  (2)

Table 3 provides the estimated densities and viscosities of DMS-V21,DMS-V33, and PDMS calculated from Equations 1 and 2 at a 15:1 w/w ratiowith Sylgard 184 curing agent.

TABLE 3 Estimated mixture densities and viscosities of silicones at 15:1w/w ratio with Sylgard 184 curing agent. Estimated Mixture EstimatedMixture Soft Material Density (kg/m³) Viscosity (cSt) DMS-V21 975 100DMS-V33 975 2,820 PDMS 1105 3,940

After mixture densities and viscosities were determined, settlingvelocity, v, was estimated using Stokes Law. Assumptions were madebefore estimating each settling velocity. These assumptions were asfollows: laminar flow where the Reynolds number is less than 0.3,homogenous mixtures, particles were spherical with smooth surfaces, andno particle-particle interactions. These assumptions combined to formEquation 3 from Stokes Law to solve for the settling viscosities ofcarbonyl iron, iron oxide, magnetite, and neodymium⁴⁴.

$\begin{matrix}{v = {\frac{2\left( {\rho_{p} - \rho_{s}} \right)}{9\mu}gR^{2}}} & (3)\end{matrix}$

Since nickel and cobalt were both cylindrical, a modified relationshipfrom Stokes Law of flow of spherical particles is formed⁴⁵. Equation 4accounts for the correlation between the cylinder's length, L, anddiameter, D.

$\begin{matrix}{v = {\frac{{0.0}790\left( {\rho_{p} - \rho_{s}} \right)}{\mu}g{L^{2}\left( \frac{L}{D} \right)}^{{- {1.6}}64}}} & (4)\end{matrix}$

FIG. 33 presents the estimated settling velocity of all sixferromagnetic particles within a varying range of viscosity from100-4,170 cSt. All of the materials except for the neodymium display asettling velocity of less than 0.40 mm/min in solutions with a viscosityof 100 cSt. Therefore, these particles were all considered to havesettling velocities near zero within PDMS during the curing process.Even iron oxide's settling velocity was approximately zero withinDMS-V21, which had an estimated viscosity of 100 cSt. However, theneodymium displays a settling velocity range from 0.60 mm/min to 25.60mm/min in solutions with a viscosity of nearly 4,170 and 100 cSt,respectively. Therefore, neodymium's settling velocity could not beignored during the curing process as it would take about 15 minutes forneodymium particles to settle within PDMS with a viscosity of 4,000 cSt.

The neodymium had an average particle size of 50-60 μm so we wanted toinvestigate the acceptable range of particle sizes within varyingviscosities so that neodymium's settling velocity could then be assumedto zero. FIG. 34 presents an estimated settling velocity for varyingparticle sizes of neodymium of 0, 25, 50, 75, and 100 μm in solutionswith a viscosity range of 100-4,170 cSt. Only neodymium particle sizesof 25 μm or less could be considered to have an assumed settlingvelocity of approximately zero in solutions with a viscosity of 2,000cSt or more.

CONCLUSIONS

We were able to identify that 1 wt % was the minimum requiredconcentration of magnetic filler to detect a magnetic response fromcompressive forces and displacement for nickel nanorods, cobaltnanowires, iron oxide nanopowder, magnetite powder, and neodymium powderincorporated within PDMS. However, this was not the case for thecarbonyl iron microspheres integrated within PDMS as there was no viablemagnetic response generated at 1 wt %.

Overall, the 1 wt % neodymium-PDMS cylindrical constructs showed thegreatest magnetic field response when compressed at 600 g of force.However, the particles were also much larger than the other magneticmaterials. Even in PDMS at a 15:1 w/w ratio of base to curing agent,which has an approximate viscosity of 3,900-4,000 cSt, the neodymiumsettled at a rate of 0.64 mm/min. Therefore, moving forward, smallerneodymium particles should be evaluated.

The PDMS also displayed the most desirable properties and relationshipbetween force and displacement with and without the magnetic fillers.Without any magnetic materials incorporated, the PDMS and DMS-V21exhibited similar Young's moduli which were the most favorable as theywere compressible, unlike the BJB, but they were also resistive enoughto not reach their ultimate strength at lower forces like the DMS-V21.However, the PDMS was more desirable than the DMS-V33 because of themagnetic field response generated when 1 wt % iron oxide wasincorporated into the materials and compressed.

Although the neodymium powder displayed the strongest magnetic fieldresponse, it was also the largest particle compared to the nickelnanorods, carbonyl iron microspheres, cobalt nanowires, and iron oxidenanopowder, magnetite powder. Therefore, they displayed a settling ratefar exceeding the other materials. Since it cannot be assumed that thesettling rate of neodymium was zero during the curing process, it wouldbe important for further investigations to assess the magnetic fieldstrength of neodymium particles at a much smaller size, on the order of25 μm or less.

Rheological analyses will be vital in understanding the ability thatthese ferromagnetic fillers have on additive manufacturing techniquessuch as 3D printing. Although a certain magnetic particle like theneodymium displayed favorable magnetic field response, it may not befavorable for 3D printing techniques such as DIW. In order to displaythe rheological prerequisites for desirable 3D printing techniques likeDIW, the inks need to be of suitable sizes as not to obstruct theextrusion heads during manufacturing. Inks also need to have a certainviscosity range of 0.1 to 1,000 Pa/s^(35, 46-48). Therefore, it would beimportant to continue analyzing different magnetic materials at alteredwt %'s and within varying viscous silicones (i.e., DMS-V21, DMS-V33, andPDMS with varying w/w ratios of base to curing agent).

Investigating these different silicones at varying w/w ratios wouldprovide important information. Our findings and those of others suggestthat even if certain ratios of each silicone composite display similarviscosities, their storage and loss moduli may still be very different.DIW inks need to have the ability to flow at a lower viscosity whenplaced under a shear force, meaning that the loss modulus needs to begreater than the storage modulus. However, it's imperative that DIW inksthen have the ability to rapidly heal after extrusion so they recoverafter the shear force is removed, which means the storage modulus is nowgreater than the loss modulus. The faster the recovery, the moredesirable the ink becomes especially since shear thinning are extremelycommon in soft materials³⁶.

Aside from utilizing these magnetically infused polymers in DIWprinting, the ferromagnetic materials can be incorporated intophotopolymerizable resins. This would explore the ability to printmagnetic structures from these resins through stereolithography (SLA)printing. Minor inquiries were made into incorporating magnetic fillersinto resins, but they became too opaque to crosslink at heights above2-3 mm using an ultraviolet lamp. Future investigations would includethe ability to an electromagnet within the SLA printer as well asprinting or crosslinking in smaller and more consistent heights of about100 μm.

Example 3: 3D Printed Silicone Cushions Using NuSil R40

The Berkland Laboratory at the University of Kansas has beeninvestigating ferromagnetic compounds in silicone cushions that may besensed remotely in order to identify those compounds with the highestpotential for commercial utilization in a number of technicalapplications such as wireless sensors. 3D printed silicone cushionsusing NuSil R40 were prepared and the magnetic properties of nickelnanorods suspended in the NuSil R40 were evaluated.

It was determined that the nickel nanorods extruded well in the siliconeinks (10 wt %, 1 wt %, and 0.1 wt % concentrations of nickel nanorods inNuSil R40) and aligned in the direction of flow (FIG. 12). NuSil R40extrusion tests were also performed using 1 wt % nickel nanorods ofvarying lengths, specifically categorized as small (1-4 μm), medium (4-8μm), and large (8-12 μm) (FIG. 13).

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Example 4: Shoe Soles to Detect the Displacement and Force

Using wireless triple axis accelerometer and magnetometer via Bluetoothor another wireless connection would allow for the possibility toincorporate sensors directly into the silicones or polymers themselves.Larger constructs could be built in 12 well plates or even petri dishesdepending on the size of the remote wireless magnetometer. There is thenthe possibility of integrating these larger constructs into shoe solesto detect the displacement and force of the person walking based on themagnetic field strength generated from the magnetic particles that arethen detected from the sensor within the composite. However, because theplacement of the sensor-containing composite in the sole faces repeatedforce and displacement, it may also be better to place the wirelessmagnetometer in close proximity to the magnetic composite (i.e., thetongue of the shoe) as to not break the sensor due to repeatedcompression of the shoe soles. In this case, it would also be importantto develop a more formal relationship between the force and displacementof the materials against their magnetic field response. That way, theforce and the displacement could be determined from the magnetic fieldresponse received by the magnetometer. This technology could be thenutilized to help predict possible injuries from repeated stress andstrain within people and potentially professional athletes.

The compositions, devices, systems, and methods of the appended claimsare not limited in scope by the specific compositions, devices, systems,and methods described herein, which are intended as illustrations of afew aspects of the claims. Any compositions, devices, systems, andmethods that are functionally equivalent are intended to fall within thescope of the claims. Various modifications of the compositions, devices,systems, and methods in addition to those shown and described herein areintended to fall within the scope of the appended claims. Further, whileonly certain representative compositions, devices, systems, and methodsteps disclosed herein are specifically described, other combinations ofthe compositions, devices, systems, and method steps also are intendedto fall within the scope of the appended claims, even if notspecifically recited. Thus, a combination of steps, elements,components, or constituents may be explicitly mentioned herein or less,however, other combinations of steps, elements, components, andconstituents are included, even though not explicitly stated.

The term “comprising” and variations thereof as used herein is usedsynonymously with the term “including” and variations thereof and areopen, non-limiting terms. Although the terms “comprising” and“including” have been used herein to describe various embodiments, theterms “consisting essentially of” and “consisting of” can be used inplace of “comprising” and “including” to provide for more specificembodiments of the invention and are also disclosed. Other than wherenoted, all numbers expressing geometries, dimensions, and so forth usedin the specification and claims are to be understood at the very least,and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, to be construed in light of thenumber of significant digits and ordinary rounding approaches.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of skill in the artto which the disclosed invention belongs. Publications cited herein andthe materials for which they are cited are specifically incorporated byreference.

What is claimed is:
 1. A system comprising a 3-dimensional articleformed from a composition comprising an elastomeric resin, and apopulation of anisotropic magnetic particles dispersed within theelastomeric resin, wherein the anisotropic magnetic particles arealigned and/or oriented within the article; and a magnetometer or a HallEffect sensor, configured to interrogate the magnetic field strengthwithin the article; and a magnet or an electromagnet, configured toapply a magnetic field within the article, wherein the strength of themagnetic field can be varied to vary a mechanical property of thearticle; and a processor configured to vary the strength of the appliedmagnetic field to induce a target mechanical property in the article. 2.The system of claim 1, wherein the anisotropic magnetic particlescomprise nanoparticles.
 3. The system of claim 1, wherein theanisotropic magnetic particles comprise rod-shaped magnetic particles.4. The system of claim 3, wherein the rod-shaped particles have anaspect ratio of from 5 to 500, or from 5 to
 250. 5. The system of claim3, wherein the rod-shaped particles have a diameter of from 50 nm to 500nm, or from 100 nm to 300 nm.
 6. The system of claim 3, wherein therod-shaped particles have a length of from 1 micron to 25 microns. 7.The system of claim 1, wherein the anisotropic magnetic particlescomprise plate-like particles.
 8. The system of claim 1, wherein theanisotropic magnetic particles are present in the composition in anamount of from 0.1% by weight to 10% by weight, based on the totalweight of the composition, or from 0.1% by weight to 5% by weight, from0.1% by weight to 2.5% by weight, or from 0.1% by weight to 1% byweight, based on the total weight of the composition.
 9. The system ofclaim 1, wherein the composition further comprises a non-magneticfiller, or silica particles.
 10. The system of claim 1, wherein theelastomeric resin comprises a crosslinkable composition, or acrosslinkable silicone composition.
 11. The system of claim 10, whereinthe elastomeric resin comprises (A) a first organosilicon compoundhaving at least two ethylenically unsaturated moieties per molecule; andoptionally (B) one or more additional organosilicon compounds.
 12. Thesystem of claim 1, wherein the article is formed by an additivemanufacturing process.
 13. The system of claim 1, wherein the articlecomprises a cushion or structural member.
 14. The system of claim 1,further comprising a processor configured to calculate a force appliedto the article based on a measurement of a change in the magnetic fieldstrength within the article.